Infrared Absorption Associated with Strong Hydrogen Bonds

Year: 1961 DOI: 10.7907/JACR-N881
The hydrogen bond systems in potassium hydrogen fluoride, acetamide hemihydrochloride, nickel dimethylglyoxime, potassium hydrogen bis-phenylacetate and maleic acid were studied via infrared spectroscopy between 5000 cm⁻¹ and 400 cm⁻¹. Each compound was studied in detail by the potassium bromide pressed disc technique.

The infrared active fundamentals of the bifluoride ion were found to be shifted from the values reported for the pure, crystalline compound. Integral absorption intensities of the two infrared active fundamentals in the KBr solid solution were evaluated and interpreted in terms of the effective charge motion.

Spectra of acetamide hemihydrochloride, nickel dimethylglyoxime and potassium hydrogen bis-phenylacetate exhibited unusual features: an intense, extremely broad background absorption between 2000 cm⁻¹ and 400 cm⁻¹ and a deep "window" in the broad background. Maleic acid, however, was found to give a normal, well-behaved spectrum. These unusual features have been interpreted in terms of the symmetry of the hydrogen bond and the complexity of the hydrogen bonded molecules. The broad background is regarded as the superposition of several hydrogen bond vibrational modes, each of which is anharmonically coupled with low frequency intermolecular vibrational modes. The "windows" have been explained on the basis of a special perturbation between certain vibrational energy levels of the hydrogen bond modes and the molecular group modes in a small fraction of the molecules; in a majority of the molecules the perturbation is either absent or is very weak. It is suggested that these features are characteristic of short symmetric hydrogen bonds in complex molecules.